The present application relates to semiconductor structures, and more particularly to field effect transistors (FETs) having multiple metal gates with different threshold voltages and methods of manufacturing the same.
In advanced semiconductor chips, multiple types of field effect transistors (FETs) with different threshold voltages are used to realize circuit function. A challenge, however, associated with integrating different types of transistors is that each type of transistor generally requires a threshold voltage that is different from what the other types of transistors require. For example, static random access memory (SRAM) transistors typically require a higher threshold voltage than logic transistors due to the relatively lower power requirements of SRAM transistors as compared to logic transistors.
In traditional planar FET technology, threshold voltage adjustment can be achieved through channel doping. Specifically, ion implantation is performed to alter the threshold voltage of SRAM transistors relative to logic transistors, and vice versa. However, when the threshold voltage of a device is increased by increasing the doping concentration in the channel region, carrier mobility decreases, and device performances deteriorate. Moreover, the highly-doped ions in the channel region may compensate the ions in the region where a source or a drain region meets the channel region, thus decreasing the doping concentration in such region and increasing the device resistance.
The conventional channel doping approach is not applicable to adjust the threshold voltage of fin FETs (FinFETs). Due to three-dimensional geometry and static electricity of semiconductor fins, channel doping in FinFET technology leads to dopant fluctuations and threshold voltage variation, which in turn causes the degradation of the device performance. Therefore, there remains a need for improved device structure and method that allow better manipulating threshold voltages for different types of FETs without degrading device performance.